The present invention relates to an ultraminiature optical deflector which uses a coreless flat DC brushless motor and is suitable for use with an optical data processing apparatus equipped with a lazer deflector, e.g. a laser printer, a digital copier and a facsimile apparatus. More particularly, the present invention is concerned with a deflector which uses a dynamic pressure pneumatic bearing as a radial bearing and a magnetic bearing as a thrust bearing so as to enhance rigidity of the magnetic bearing associated with a lifting force by simple means. Such a deflector is operable stably against unusual external disturbances and achieves an extended service life with wear due to contact of a rotary body being eliminated.
Various kinds of optical data processing apparatuses which are equipped with a laser deflector are known in the art, e.g. a laser printer, a digital copier, a facsimile terminal, and an image scanner of a POS terminal. Equipments generally used to scan bills, ion sheets and others to locate their defects and other various kinds of optical equipments such as for measuring lengths are also provided with an optical deflector. Typically, any of such laser or optical deflectors is implemented with a polygon mirror. A deflector of the type using a polygon mirror is capable of reading or writing information rapidly and accurately since it deflects a light beam at high speed and, yet. continuously.
However, the problem with a picture output terminal with such a laser deflector, e.g. a laser printer is that a picture involves jitter due to irregular rotations of the polygon mirror.
Optical deflectors known in the art include a magnet field system DC brushless motor type deflector in which a thrust magnetic bearing for a rotary body is implemented with a bearing of the kind utilizing repulsive forces of permanent magnets. Specifically, in such a particular type of deflector, a rotor of a DC brushless motor is constituted by a cylindrical magnet, and a hollow shaft provided with a rotatable polygon mirror is rigidly connected to the inner periphery of the rotor magnet. A stationary shaft is disposed inside the hollow rotary shaft, or so-called polyhedron, so that the outer periphery of the stationary shaft cooperates with the inner periphery of the polyhedron to constitute a dynamic pressure pneumatic bearing. A polygon mirror type deflector implemented with such a dynamic pressure pneumatic bearing is constructed such that when a power source is turned on the rotary body with a rotary polygon mirror or the like begins to rotate. Upon lapse of several seconds, a thrust pneumatic bearing set up beforehand becomes effective so that the rotary body reaches a predetermined rotation speed which allows it to lift itself while sustaining an axial load thereof. Under this condition, air flows into and from below the gap defined between the inner periphery of the polyhedron and the outer periphery of the fixed shaft and, due to a pumping function of herringbone grooves formed in an upper and a lower journals, serves to firmly support the rotary body in the radial direction. While supporting the rotary body so, the air flows upwardly due to the effect of spiral grooves so as to exert a pressure upwardly on a thrust stop portion and, then, flows further upwardly out of the dynamic pressure pneumatic bearing via a through opening of the thrust stop portion. By such a procedure, the rotary body which carries the polyhedron therewith is rotated.
A prior art deflector with the above-described kind of pneumatic bearing has various drawbacks as will be described hereinafter.
First, the pneumatic bearing section, e.g., outer periphery of the stationary shaft requires complicated grooving work while the inner periphery of the elongate polyhedron, too, needs very accurate surface machining. For these reasons as well as others, machining processes are difficult to perform and, therefore, add to the production costs.
Second, concerning a dynamic pressure pneumatic bearing, the bearing section is held in a contact state and, hence, the pneumatic bearing effect is not available at the time of a start with the result that a long life span, which is the most significant advantage of a pneumatic bearing, is unachievable. Specifically, the bearing section wears at each time of start and stop of operation due to frictional contact thereof.
Third, the wear of the bearing section is aggravated since the motor is of an inner rotor magnetic field system type, i.e., since a rotor is constantly attracted toward a core type stator while the motor is out of operation.
Fourth, the cost situation discussed above as the first problem becomes more severe since the construction of the rotary body is complicated and the number of structural elements is great to render balance correction difficult.
Fifth, the inner rotor type motor is inherently small in inertia to degrade the picture jitter characteristic and, at the same time, makes the deflector bulky due to increase in the length of the rotary shaft.
Sixth, the inner rotor magnetic field system DC brushless motor has to be accompanied by a velocity detector which adds to the intricacy of construction of the stator side as well as to the number of structural elements. The result is the deterioration of reliability and, again, increase in cost.